Morphology of a planer precipitate and its orientation relationship to the matrix in an Al-1.0 mass%Mg₂Si-0.4 mass%Si alloy

The morphology of planer precipitates and the orientation relationships between the precipitates and the matrix in an Al-1.0 mass% Mg₂Si-0.4 mass%Si alloy were investigated by transmission electron microscopy (TEM). Some equilateral triangular precipitates and many polygonal precipitates were observed by a scanning electron microscope. TEM observation of the precipitates and the extracted precipitates from the matrix by thermal phenol method shows that the equilateral triangular precipitates are thin plates. The electron diffraction patterns taken from these precipitates were analyzed as a silicon that has a diamond structure (a = 0.543 nm). Energy dispersive X-ray spectroscopic (EDS) analysis measured at the extracted precipitates showed only Si peak. Some straight boundaries exist in the equilateral triangular precipitate. It was observed by high resolution electron microscopy that the boundaries are plane defects such as stacking faults. Electron diffraction spots taken from this precipitate show the forbidden reflection. Such spots are caused by stacking faults. Therefore, the equilateral triangular precipitate is silicon. Orientation relationships between the precipitate and the matrix is as follows; (001) _Al//(111)_Si and the angle between [100]_Al and [1̄10]_Si is 10 degrees. Three sides of the triangular Si precipitate are parallel to the direction of 〈110〉_Si in (111)_Si.